There is a fundamental problem in aviation that until recently was not recognized as being the source of pilot error. The problem is that it is extremely difficult to subjectively determine visibility range during flight accurately. As such, pilots are prone to fly into poor visibility situations and not realize it until it is too late. For general aviation such as small planes, there typically is no equipment in the aircraft to allow for instrument flying (because of the cost of the equipment), which results in the pilot gambling he/she will be lucky and is able to fly into better visibility conditions before crashing. This is less of an issue on larger aircraft, because they typically operate under instrument flight rules since these aircraft have equipment that allows instrument flying when visibility decreases and/or becomes difficult to judge such as at night and/or over large bodies of water.
Every year, military and civilian aviation lose lives and aircraft due to the spatial disorientation experienced during periods of minimal visibility or inadvertent entry into instrument meteorological conditions. The flights sometimes end catastrophically when the aircraft flies into an unseen terrain such as a mountain or other unyielding surface.
Spatial disorientation occurs “. . . when the aviator fails to sense correctly the position, motion, or altitude of his aircraft or of himself within the fixed coordinate system provided by the surface of the earth and gravitational vertical.” Benson, Spatial Disorientation: General Aspects, Aviation Medicine, 1978. Spatial disorientation remains an important source of attrition in aviation. U.S. Army Field Manual 3-04.301 (Department of the Army, 2000), Aeromedical Training for Flight Personnel, states that, “[s]patial disorientation contributes more to aircraft accidents than any other physiological problem in flight.” Regardless of their flight time or experience, all aircrew members are vulnerable to spatial disorientation. According to a Federal Aviation Administration technical report (Kirkham et al., Spatial Disorientation in General Aviation Accidents, FAA Civil Aeromedical Institute, Report No. FAA-AM-78-13, 1978), for all fatal accidents in small fixed-wing aircraft from 1970 through 1975, 22.2% involved continued flight into adverse weather while operating under VFR (visual flight rules) and 16.4% were attributed to spatial disorientation. According to the U.S. Army Safety Center (USASC) accident files and a report published by the U.S. Army Aeromedical Research Laboratory (USAARL) (Braithwaite, et al., Spatial Disorientation in U.S. Army Helicopter Accidents: An Update of the 1987-92 Survey to Include 1993-95, U.S. Army Aeromedical Research Laboratory, USAARL Report No. 97-13, 1997), spatial disorientation was considered to be a significant factor in 291 (30 percent) of Class A, B and C helicopter accidents in the U.S. Army between 1987 and 1995. According to the report, during this time, 110 lives were lost and a cost of nearly $468 million was incurred. The monetary cost of spatial disorientation is high and the fatality rate is between one and one-half to two times that of nondisorientation accidents.
Preliminary results of a review of spatial disorientation accidents for fiscal years (FY) 1996 through 2000 showed similar trends with reviews by Durnford et al., Spatial Disorientation: A Survey of U.S. Army Helicopter Accidents 1987-1992, U.S. Army Aeromedical Research Laboratory, USAARL Report No. 95025, 1995 and Braithwaite, et al. (1997). It was further stated that data comparison with fiscal years 1991 through 1995 showed that the spatial disorientation accident rate is not decreasing, and if anything, since 1995, has slowly started increasing. This trend indicates that despite the best efforts of the USASC to educate the aviator through printed accident reviews and the efforts of the developers of improved aircraft orienting technology (cockpit head-up displays, improved night vision devices, global positioning navigation systems, etc.), there has been little change in the spatial disorientation accident rate.
Over the past six years, weather and spatial disorientation has caused 21% of all accidents and 49% of the fatal crashes involving lifesaver flights. Springer, The IFR Bullet, Air Medical Journal, Vol. 24, No. 1, January-February 2005. It is estimated that if the weather related accidents were eliminated, then the accident rate (per 100,000 hours) would go from 6 down to 4 while cutting the fatal crash rate in half from 2 to 1.
In the last two years, there was a multiple helicopter NVG long range surveillance corps extraction training insertion conducted by the U.S. Army. The terrain flight was under zero illumination, no visible horizon and unknown weather, which led to a situation involving spatial disorientation and a controlled flight into terrain resulting in four fatalities and a total cost of $8.4 million.
An important action required by pilots in order to maintain situation awareness and avoid visual conditions likely to cause spatial disorientation is to correlate actual enroute visibility with the minimum visibility required for a particular class of airspace or with a mission's minimum visibility as an abort criterion.
Visibility is one of the most complicated of all meteorological elements to determine during flight. The measure of visibility and visual range depends on the characteristics of the atmosphere, the type of viewing instrument, the type of object or light being detected, and the manner by which the object or light is being viewed. The primary factors influencing visibility include: reflecting power and color of the object, reflecting power of the background, amount of scattering and absorbing particles, position of the sun, angular size of the object, nature of the terrain between the object and observer, contrast of the object, and intensity of the light source.
In the case of classes of airspace that allow VFR flight as defined in the Federal Aviation Regulations (U.S. Government, 2003), when flying VFR, it is incumbent on aviators to maintain at least the minimum visibility required for that airspace. The Federal Aviation Regulations issued by the Federal Aviation Administration (FAA) establish rules under which pilots must operate depending on the class of airspace through which they are flying. U.S. pilots must comply with host nation rules when operating in foreign airspace. In the continental United States, there are six classes of airspace that depend upon the class of aircraft, the altitude being flown, and geographical location. In addition to vertical and horizontal dimensions, a regulatory factor of these classes of airspace is the minimum visibility required within each specific class in order to remain legal. As an example, any pilot flying under VFR in Class G airspace below 1200 feet above ground level (AGL) must maintain 0.5 statute mile (sm) visibility during the day (1 statute mile at night) to comply. In a combat/tactical situation, where flight operations are not subject to FAA regulations or host nation rules, the aviation mission commander establishes the minimum visibility requirement.
It is, therefore, incumbent on the pilot to determine if he/she is complying with the relevant regulations by estimating the predominant visibility during the conduct of a VFR flight. Because these visibility estimates (subjective assessments) vary widely from pilot to pilot, some pilots actually overestimate the predominant visibility. This overestimation causes some pilots, at a minimum, to violate regulations. At worst, it may lead a pilot to become spatially disoriented due to the loss of visual references and can result (and has resulted) in a CFIT (controlled flight into terrain) accident.
During tactical missions, especially at night involving multiple aircraft, when civil visual flight rules may not apply or may be too lenient, an aviation unit commander, or his/her representative, must establish a set of criteria that requires the mission to be aborted should any of the criteria be met during the conduct of the mission. Examples of these criteria are maximum wind velocities, minimum cloud levels, enemy detection and concentrations, equipment malfunctions, and atmospheric visibility. The commander bases these criteria on objective and subjective assessments of his/her unit. Hence, different units have different criteria. Obviously, if a mission required at least three aircraft and all but two malfunctioned, the mission abort criterion for the number of aircraft would be met and the mission would have to be aborted (objective criterion). On the other hand, the commander must establish subjective criteria, also. In establishing these subjective abort criteria, the commander, for example, would determine the minimum/maximum conditions under which he/she believes that the aircrews of that unit would be able to successfully complete a given mission. Certainly, a highly trained, experienced, group of aviators would be able to perform and complete a mission under more difficult and demanding conditions than a group of less experienced aviators. Frequently, these mission abort criteria are incorporated in the unit's standing operating procedures (SOPs) and are standardized for consistency, clarity and brevity. These criteria always, and necessarily, include the minimum atmospheric visibility required for a mission.
Therefore, it is a duty of the pilot-in-command in a single-aircraft mission or of the air mission commander in a multi-aircraft operation to determine the prevailing visibility (greater than 180° of the horizon), or at least the visibility of the sector through which they are traveling (a 45° arc of the horizon circle), during the conduct of a mission to ensure that the minimum visibility for that mission has not been exceeded. U.S. Army rotary-wing pilots have always had to use their judgment and experience to subjectively assess the enroute visibility during a flight. Every aviator has struggled at some point during his/her career to meet VFR and/or mission minimum visibility requirements. As weather deteriorates, the pilot must rely on his/her subjective analysis to formulate a course of action: to proceed, alter, or abort the mission. Often, the aircrews proceed into these potentially dangerous conditions, not because the crews are negligent or irresponsible, but because of an honest effort to accomplish the mission and because there is no sure way to know the exact visibility during the flight, especially at night and/or while using night vision goggles (NVGs).
The pilot's ability to estimate visibility during flight comes through experience. During initial pilot training, the aviator does not receive a formal course of instruction in estimating atmospheric visibilities. He/she learns this skill through mentorship and trial and error. Most aviators rely on the ability to see a known object through the visual obscuration and attempt to judge the distance, which is extremely difficult at night and nearly impossible when flying over large bodies of water. Some aviators use relative distances and/or map cross-referencing (plotting one's position and measuring distance to the visual object). These estimates can vary widely from one aviator to another within the same aircraft or from aircrew to aircrew within a multi-aircraft formation.
The difficulty in estimating visibility with any degree of accuracy is due to the many variables involved. Of course, the absence of standardized formal training is one contributing factor. Other factors include the pilot's own visual acuity and contrast sensitivity of the pilot's eye, and the variance in the contrast sensitivity causes a large variance in estimation of visibility. It is well known that contrast sensitivity is impacted by the angular dimension and brightness, on the age and degree of training of the observers and a number of other factors. Although small in aviation, some differences do exist in each aviator's ability to focus and perceive distant images. Additionally, some pilots are more inclined to make rapid judgments based on their perceptions while others are more patient and thoughtful. The complicating effects of the NVGs, due to their monochromatic nature (shades of green), cause contrasts to be less noticeable. NVGs' ability to amplify light allows some obscurations to be easily “seen through” thus reducing the contrast sensitivity in what the pilot sees.
As a group, it is likely that meteorologists are the most experienced and proficient at estimating prevailing and sector visibility using eyesight. Visibility observations are made on the basis of normal vision, i.e., without the aid of optical devices such as binoculars or telescopes. Observations are made at an eye level of six feet above the ground (an internationally recommended practice). Observers are to select markers (objects) such as buildings, chimneys, hills, trees, and towers that are at verified distances and, thus, if the object is visible, there exists at least that distance of visibility. Although the above-described procedure is accepted as accurate, the reported areas are limited to those areas that are actually observed and, for example, are not necessarily predictive of the areas along a 100-mile flight path. Because these observations are determined by human evaluation, the information itself may be flawed due to different contrast sensitivities.
In order to eliminate the inherent human errors delineated previously and to provide objective visibility data, instruments can be, and are, used to determine visual ranges. According to Lujetic, A Report on Atmospheric Obstructions to Visibility, U.S. Army Engineer Topographic Laboratories, Report No. ETL-0169, 1979, there are a number of land based instruments developed and used for the measurement of visual range. The basic attenuation mechanisms are scattering and absorption. Scattering is the process by which a particle in the path of an electromagnetic wave continuously abstracts energy from the path incident wave and reradiates that energy. Absorption is the process by which agents in the atmosphere abstract energy from a light wave. These measuring instruments can be separated into general categories: 1) those that measure the scattered light by sampling a small volume of air using a source and receiver, and 2) those that determine the transmittance of a path of known length using a light source and a telephotometer. Back-, side-, and forward-scatter meters are examples of the first type; and transmissometers are examples of the second.
The instruments above are generally used in fixed applications, i.e., at airports or weather observing stations. In other words, these instruments are stationary and measure the visual range (presence of obscurations) over fixed distances. In applications involving a moving instrument, however, as would be necessary if mounted to an aircraft operating at high airspeeds, other instrument configurations and capabilities must be considered. The U.S. Air Force has offered the opinion that as with fixed applications, the instrument would have to have the capability of determining the visual range under many conditions of precipitation (drizzle, rain, snow, snow grains and pellets, ice crystals and pellets, and hail) and obscurations (mist, fog, smoke, volcanic ash, dust, sand, and haze).
The term LIDAR is an acronym for light detection and ranging. LIDAR systems employ intense pulses of light, typically generated by lasers, and large telescopes and sensitive optical detectors to receive the reflected pulses. They are most commonly used to measure the composition and structure of the atmosphere, such as by tracking weather balloons, smoke puffs, reflections and scattering effects of clouds, and rocket or aircraft trails. The very narrow beam width, narrow line width, and ultra short pulses of the laser make it possible to optically probe the atmosphere with exceptional sensitivity and resolution.
A LIDAR system essentially consists of three main components: a transmitter to emit a laser light, a receiver to collect the backscattered radiation, and converter electronics to produce an interpretable signal from the collected amount of light.
Even though LIDAR has been used by the U.S. Air Force and NASA to evaluate a wide variety of atmospheric properties such as turbulence, pollution and wind shear, and the existence of contrails behind aircraft. To the knowledge of the inventors, there is not a commercially available LIDAR system for determining visibility ranges from aircraft.
Without objective criteria, a pilot will want to do everything in their power to complete a flight or mission, and may as a result push the visibility envelope to avoid abandonment of the flight or mission based on a subjective measure that the pilot was never trained to make but only learned through experience. As such, a need exists to provide an objective measure regarding visibility and the ability to have a pilot alert mechanism when the current visibility has deteriorated below a predetermined visibility threshold.
Notwithstanding the usefulness of the above-described systems and methods, a need still exists for an objective determination of visibility for use by a flight crew to determine whether they should abort a particular flight plan and/or mission.